Cathode ray tube deflecting circuit



y 9, 1940- K. R. WENDT 2,207,389

CATHODE RAY TUBE DEFLECTING CIRCUIT Filed June 30, 1936 3nventorPatented July 9, 1940 UNITED STATES PATENT orrlca Karl R. Wendt,Audubon, N. J., assignor to Radio Corporation of America, a corporationof Delaware Application June 30, i936, Serial No. 88,143

11 Claims.

My invention relates to cathode ray tube deflecting circuits, andparticularly to deflecting circuits for cathode ray tubes of the typehaving a screen structure inclined with respect to the electron gun.

An object of my invention is to provide improved means for correctingthe keystone scanning pattern appearing on the screen structure in acathode ray tube of the above-mentioned type. More specifically, anobject of my invention is to provide a simplified, inexpensive, andeasily adjusted deflecting circuit which will produce a substantiallyrectangular scanning pattern in a cathode ray tube of theabove-mentioned type.

A still further object of my invention is to provide an improvedgenerator of saw-tooth waves.

A still further object of my invention is to provide a simplifiedcircuit for producing sawtooth waves in the output circuit of apush-pull amplifier or modulator.

In practicing my invention, I employ a pushpull or balanced modulatorwhich is so designed that it functions both as a linear amplifier and uas a linear modulator. The modulator comprises vacuum tubes having highplate impedance, these tubes preferably being of the screen-grid type.Transformer coupling is employed both in the input circuit and in theoutput circuit of this a modulator, the primary winding of the inputtransformer being utilized as a portion of the saw-tooth generator. Inthe output circuit, at least one winding of the transformer is shuntedby a resistor having such low impedance that the vacuum tubes feed intoa substantially purely resistive load. Also, this load impedance is madeso low compared with the plate impedance of the screen grid tubes thatthey function as a substantially linear modulator.

Other objects, features, and advantages of my invention will appear fromthe following description takenin connection with the accompanyingdrawing, in which Figure l is a circuit diagram of a preferred vembodiment of my invention, and

Figures 2 and 3 are diagrams which are referred to in explaining theinvention.

Referring to Figure 1, my invention is shown applied to the deflectingcircuits for a cathode ray transmitter tube in a television transmitter.The transmitter tube is of a well known type comprising an evacuatedenvelope I having therein an electron gun consisting of an indirectlyheated cathode 2, a control grid 3, and

a first anode 4. The tube also includes a second anode 8 and a mosaic Iof light sensitive elements upon which the picture to be transmitted isprojected.

Preferably, the cathode ray tube is provided with horizontal deflectingcoils 8 and vertical 6 deflecting coils 9, although deflecting platesmay be substituted if desired. It will be understood that if deflectingplates are employed the output circuit of the deflecting circuit will besuitably modified. It will also be understood that the 10 cathode raytube may be a receiver tube in which the screen structure is afluorescent screen instead of a light sensitive mosaic.

Whether the cathode ray tube is a transmitter tube of the typeillustrated or a receiver tube M of the type in which the picture isviewed from the side of the fluorescent screen being scanned, the screenstructure is inclined with respect to the electron gun as shown in thedrawing, whereby the pattern scanned on the screen structure by thecathode ray normally has a keystone shape as indicated at H in Fig. 2.The desired scanning pattern, of course, is a rectangular one, asindicated at l2 in Fig. 3.

The desired rectangular scanning pattern may 25 be obtained by graduallyincreasing the amplitude of the comparatively high frequency, horizontalsaw-tooth waves as the cathode ray is deflected from the top edge of thescreen I to the bottom edge thereof by the comparatively 30 lowfrequency, vertical saw-tooth waves. Of, ii the top edge is nearer theelectron gun than the bottom edge, this amplitude will be graduallydecreased as the cathode ray is deflected downwardly. The manner inwhich this correction 35 is accomplished by my improved circuit will beunderstood from the description which follows.

Referring first to the horizontal deflecting cir- I cuit which isdesigned, in a preferred embodiment, to produce saw-tooth wavesoccurring at o the frequency of 10,290 per second, it includes ablocking oscillator comprising a three-element vacuum tube l3 and afeed-back transformer H which preferably has an iron core. The secondaryl6 of the transformer I4 is connected in 5 the grid circuit of the tubel3 in series with a grid condenser l1, while the primary [8 of thetransformer is connected in the plate circuit of the tube, the directionof coupling being such that when the plate current increases the grid 50of the tube I3 is made more positive. The blocking oscillator itself iscompleted by a variable grid resistor l9 connected between the grid andthe cathode of the tube l3.

The relative values of the grid condenser ll 5 and the grid resistor l9are made such that the operation is as follows: Assuming theconditionwhen voltage is.flrst applied to the plate of the of platecurrent is prevented until such time as a portion of the charge leaksoff the'grid condenser I'I through the grid resistor l9. After enough ofthe charge has leaked off the condenser,

plate current again flows and the operation is repeated.

It will be noted that in the grid circuit of the tube I3 a resistor 2|is included in series with the transformer secondary l6. This resistoris included in the circuit to provide a simple way of impressing ahorizontal synchronizing impulse upon the blocking oscillator. Thisimpulse is supplied through a condenser 22 and'a resistor 23 from asuitable synchronizing generator in the case of a transmitter, or from aportion of the receiver in the case of a television receiver.

As will be more fully described later, my deflecting circuit includes apair of screen grid tubes 24 and 26 connected in push-pull or balancedrelationship, these tubes having a transformer 21 in their inputcircuit. The Y transformer has a primary 28 and a secondary 29, theprimary being connected in the plate circuit of the blocking oscillatortube l3 and in series with the primary l8 of the feed-back transformer.

The description of the operation of the blocking oscillator given abovewas made with the assumptionthat the voltage was applied to the plate ofthe blocking oscillator tube l3 directly through the primary l8 of thefeed-back transformer rather than through the two primaries l8 and 28 inseries. The result of such operation would be to produce a sharppositive impulse periodically, which impulse, in the more usualsaw-tooth generators, is employed to drive the grid of a discharge tubepositive periodically to produce saw-tooth waves.

In accordance with one feature of my invention, I cause a saw-toothvoltage wave to appear across the primary winding 28 of the transformer21. I accomplish this by connecting a condenser 3| acres the primarywinding 28 whereby they are resonant at a frequency which is much lowerthan the frequency at which it is desired to produce the saw-toothwaves. It has been found that if this resonant period is about V of thefrequency at which the saw-tooth waves are produced, the results will besatisfactory.

It may be mentioned that the condenser 3| may be connected eitherdirectly across the primary winding 28 or between the high voltageterminal of the primary winding 28 and ground, as shown, whereby it isactually connected across both the primary winding and the plate batteryor other plate voltage supply (not shown). The results obtained are thesame in both cases, the impedance of a plate battery or other voltagesupply being very low at the frequencies involved.

The operation of the circuit in producing a saw-tooth voltage waveacross the transformer primary 28 may be explained as follows: Thecondenser 3| is charged at a comparatively slow rate through thetransformer primary 28 by means of the plate voltage'supply (not shown).The condenser 3| is charged slowly because of the substantial impedanceof the primary winding. At the end of a certain period, the grid of theblocking oscillator tube I3 is driven positive, as previously explained,whereby the plate impedance of the tube becomes low enough to permit thecondenser 3| to discharge suddenly through the tube. It may be notedthat the impedance of the primary winding l8 of the feed-backtransformer is so low that the condenser is not prevented fromdischarging rapidly.v If the condenser 3| isconnected directly acrossthe primary 28, the condenser is charged suddenly through the tube l3and discharged gradually through the primary 28,

The broad feature of employing in the plate circuit of a vacuum tube atuned circuit resonant at a frequency which is low compared with the ffrequency of a desired saw-=tocth wave is de= scribed and claimed inPatent No. 2,085,402, issued June 29, 1937, to Arthur W. Vance andassigned to the Radio Corporation of America.

Referring now more specifically to the balanced modulator, the tube 24includes an indirectly heated cathode 32, a control grid 33, a screengrid 34,-a suppressor grid 36, and a plate 31, while the tube 26includes an indirectly heated cathode 38, a control grid 39, a screengrid 4|, a suppressor grid 42, and a plate 43. The secondary'winding29of the transformer 21 has one terminal connected to the control grid 33of the tube 24 and the other terminal connected to the control grid 39of the tube 28. The mid point of the secondary 29 is connected through aconductor 44 and a variable tap 46 to a point on a resistor 41, througha portion of the resistor 4! to ground, and through ground and aresistor 48 to the cathodes 32 and 38 which are connected together. Theupper and lower halves of the transformersecondary 29 are shunted byresistors 49 and 5|, respectively, for the purpose of preventingundesired resonant effects caused by distributed capacity in thetransformer.

The coupling between the output circuit of the balanced modulator andthe output tube 52 of the horizontal deflecting circuit is made by meansof a transformer 53 having a primary winding 54 and a secondary winding55. The primary winding 54 has one terminal connected to the plate 31 ofthe tube 24 and the other terminal connected to the plate 43 of the tube23. The mid-point of the primary winding 54 is connected to a suitablesource of plate potential (not shown). The screen grids 34 and 4| areconnected to a point on a potentiometer including resistors 58 and 51connected between the cathodes 32 and 38 and the positive terminal ofthe plate supply (not shown) whereby they are maintained at a suitablepositive potential.

The upper and lower halves of the primary winding 54 have resistors 58and 59, respectively, connected thereacross. These resistors, togetherwith a resistor 6| connected across the secondary winding 55, are forthe purpose of making the load impedance of the balanced modulatorsubstantially purely resistive, whereby the modulator will providelinear amplification, and for the purpose of providing a load impedancewhich is so low compared with the plate impedance of the modulator tubesthat the modulator will provide linear modulation. It will be understoodthat the resistance across the secondary 55 is reflected into themodulator plate circuit to determine the load impedance to a certainextent. The various considerations which determine the use of resistorsacross the transformer windings 54 and 55, and which determine theirvalues, will be discussed in more detail later in the specification.

The output tube 52 is of the screen grid type, having a high plateimpedance whereby a sawtooth voltage wave applied to its input circuitwill cause a flow of saw-tooth current through the deflecting coils 8.The tube 52 is properly biased to function as the usual Class Aamplifier, the biasing being provided by means of a selfbias resistor 62and condenser 83, in the particular circuit illustrated. The saw-toothvoltage wave which appears across the resistor BI is impressed upon theinput circuit of the tube 52 through a variable tap 64, whereby theamplitude of the horizontal deflecting current may be adjusted. Theoutput circuit of the tube 52 is coupled through an output transformer66 to the horizontal deflecting coils 8.

It will be understood that a low impedance output tube may be employedin place of the high plate impedance tube 52, but if such a substitutionis made it is necessary to supply to the input circuit of the balancedmodulator a deflecting wave which includes an impulse component. It isobvious that, in this case, both the saw-tooth component and the impulsecomponent will be modulated by the low frequency vertical sawtooth waveto provide the desired keystone correction.

Referring now to the vertical deflecting circuit which supplies asaw-tooth current to the vertical deflecting coils 9 and a saw-toothvoltage to the balanced modulator, it comprises a blocking oscillator 61of the same type employed in the horizontal deflecting circuit. Thisoscillator is adjusted to produce impulses occurring at a comparativelylow frequency, sixty per second in the particular circuit beingdescribed. By means of a condenser 68 connected across the primary 69 ofa transformer l I, these impulses are converted into saw-tooth voltagewaves which appear across both the primary 69 and the secondary 12 ofthe transformer H. In this circuit the condenser 68 is charged throughthe oscillator tube instead of being discharged through it.

The blocking oscillator 61 differs from the one previously described inthat the grid condenser 13 and secondary winding 14 of the feed-backtransformer are connected in series with the plate resistor 16 of thepreceding amplifier tube 11. The grid condenser charging circuit may betraced from the grid of the oscillator tube, through the secondary 14,the grid condenser I3, the plate resistor 16, and a by-pass condensershunting a portion of a voltage divider 18 to ground, and through groundto the cathode of the oscillator tube. Thus the vertical synchronizingimpulses are impressed upon the blocking oscillator 61 through the plateresistor 16.

A resistor 19 and condenser 8| are connected in series across thesecondary" for the purpose of straightening the saw-tooth wave, thiscorrection of the wave shape being provided because of the diificulty ofmaking the inductance of the primary winding 69 great enough to producean undistorted saw-tooth at the low frequency involved.

The vertical saw-tooth wave is supplied from the secondary 12 to theresistor 41, whereby it is impressed upon the control grids 33 and 39 ofthe modulator tubes 24 and 26 to provide the desired modulation. As iswell understood, this modulating voltage will not appear in the outputof the balanced modulator, since it is balanced out by the circuitconnections. Therefore, the only signal appearing in the-output circuitof the balanced modulator will be'the modulated horizontal saw-toothwaves, which are so modulated that as the cathode ray is deflecteddownwardly their amplitude will gradually be increased to provide on thescreen I the rectangular scanning pattern shown in Fig. 3. If any of thevertical saw-tooth waves appeared in the modulator output, the sides ofthe scanning pattern would either be slanted and parallel to each other,or otherwise distorted depending upon whether all or only a portion ofthe vertical saw-tooth frequency components are passed by the horizontaldeflecting circuit.

The saw-tooth wave appearing across the transformer primary 69 isimpressed across a resistor 82 and supplied through a variable tap 83 tothe input electrodes of an output tube 84. The output circuit of thetube is coupled to the vertical deflecting coils 9 through a suitabletransformer 86.

It may be pointed out that the requirements for a balanced modulator inmy keystone correcting circuit are rather severe. First, the modulationshould be practically linear for at least thirty percent modulation;second, the horizontal wave shape must not be distorted; and third, thecircuit must not introduce transients following the return line period.

The required linear modulation is obtained by selecting a type of screengrid tube, such as a variable mu pentode, which has a straight and steepportion on its mutual conductance, controlgrid voltage curve. By makingthe load impedance of such tubes very low compared with the plateimpedance of the tubes, the gain of the tubes will vary directly as themutual conductance, and consequently, the tubes will provide linearmodulation, the modulation being linear up to about fifty percentmodulation in the abovedescribed circuit.

It may be noted here that the gain of a screen grid tube such as the 6K7is so great that the loss in amplification, resulting from having theload impedance low compared with the tube impedance, is immaterial,there being much more gain available than is required. In ordinarymodulator practice, it is, of course, customary to make the loadimpedance much higher than the plate impedance, whereby the mostefiicient operation is obtained even though there is a resultingdistortion in the modulation. In a radio transmitter, for example, theload is a tuned circuit, and harmonic frequencies introduced bydistortion can be filtered or tuned out so that the modulationdistortion is not detrimental. In applicants circuit such filtering outof undesired frequencies, introduced by modulation distortion. isimpossible, since these frequencies fall within the range of thefrequencies being transmitted.

The requirement that the horizontal wave shape shall not be distorted iscomplied with by shunting the transformer with resistance of such lowvalue compared with the transformer inductance that the modulator feedsinto a substantially purely resistive load. Therefore, the loadimpedance remains substantially constant over a wide frequency band, andlinear or undistorted amplification is obtained over this frequencyrange. It may be noted that the load should so closely approximate apurely resistive load that secondary. This is prevented by shunting there-' sistor 6| across the secondary. This resistance then, of course,effectively introduces resistance in the primary circuit to provide thedesired resistive load for complying with the first two requirements.Resonance between the primary distributed capacity and the secondaryleakage inductance is prevented by providing the resistors 58 and 59across the primary winding.

Attention is called to the fact that no by-pass condensers are usedaround the self-bias resistor 48 or around the screen grid resistors 56and 51 in the balanced modulator circuit. This improves the linearity ofthe modulation and tends to cancel out any unbalance resulting fromadifference in the two tubes. It also degenerates the verticalsaw-tooth, making necessary more modulating voltage, but'the requiredvoltage is still less than the voltage available.

It will be understood that other types of sawtooth wave generators thanthe specific generator described may be employed in combination withmybalanced keystone correction circuit. In the event of suchsubstitution, the primary of the input transformer, of course, will notfunction as a portion of the saw-tooth generator.

If the output tube in a deflecting circuit has a plate impedance whichis low enough, only a voltage impulse should be impressed upon the inputtube of the balanced modulator, a saw-tooth component being unnecessary.In that case, the low frequency voltage wave modulates the impulsesoccurring at the horizontal frequency, instead of modulating thesaw-tooth waves occurring at this frequency.

On the drawing the values of certain resistors and condensers have beenindicated in ohms, microfarads and micromicrofarads.

From the foregoing description, it will be apparent that variousmodifications may be made in my invention without departing from thespirit and scope thereof, and I desire, therefore, that only suchlimitations shall be imposed thereon as are necessitated by the priorart and set forth in the appended claims.

I claim as my invention:

1. A deflecting circuit for a cathode ray tube having a cathode raydeflecting device associated therewith comprising means for producingvoltage waves occurring at a comparatively high frequency, a balancedmodulator comprising a pair of electric discharge tubes, said modulatorincluding an input transformer and an output transformer, means forsupplying said voltage waves to said input transformer, means forcoupling said output transformer to said deflecting device, means for soresistively loading said output transformer that said tubes work into asubstantially purely resistive load having an impedance which is lowcompared with the plate impedance of said tubes, whereby said modulatorwill provide linear amplification and linear modulation, means forproducing saw-tooth waves at a comparatively low frequency, and meansfor so impressing said saw-tooth waves upon said modulator that theymodulate said. high frequency voltage waves whereby only said modulatedwavesv appear in the modulator output.

2.A deflecting circuit for a cathode ray tube of the type having ascreen structure inclined with respect to the electron gun and having avcathode ray deflecting device associated therewith, said circuitcomprising means for producing saw-tooth waves at a comparatively highfre- I quency, a pair of electric discharge tubes cennmted in balancedrelation with an input transformer and an output transformer, means forso resistively loading said output transformer that said tubes work intoa load impedance which is low compared with their plate impedancewhereby said modulator will providelin'ear amplification and linearmodulation, means for impressing said saw-tooth waves upon said inputtransformer, means for producing saw-tooth waves occurring at acomparatively low frequency, means for varying the gain of said tubes inaccordance with said low frequency saw-tooth waves and therebymodulating said high frequency saw-tooth waves, and means for supplyingto said deflecting device the modulated high frequency saw-tooth wavesappearing in said output transformer.

3. In combination, a cathode ray tube having a screen inclined at anacute angle with respect to the axis of the electron gun, horizontaldeflecting means for deflecting the cathode ray at right angles to theplane of said acute angle, vertical deflecting means for deflecting theoathode ray in the plane of said acute angle, a horizontal deflectingcircuit including a pair of electric discharge tubes each having highplate im pedance, means including an input transformer and an outputtransformer for connecting said tubes in push-pull relation to form abalanced modulator, means for so resistively loading said outputtransformer that said tubes work into a substantially purely resistiveload having an impedance which is low compared with the plate impedanceof said tubes whereby said modulator' will provide linear amplificationand linear modu-' lation, means for supplying saw-tooth waves occurringat a comparatively high frequency to said input transformer, a verticaldeflecting circuit modulated by said low frequency waves, and means forsupplying said modulated waves to said horizontal deflecting means.

4. In combination, a pair of electric discharge tubes of the screen gridtype each having a control grid and a. plate, means including an inputtransformer and an output transformer for connecting said tubes inpush-pull relation to form a. balanced modulator, means for soresistively loading said output transformer that said tubes work into asubstantially purely resistive load having an impedance which is lowcompared with the plate impedance of said tubes whereby said modulatorwill provide linear amplification and linear modulation, means forproducing sawtooth waves occurring at a comparatively high frequency andfor impressing them upon said input transformer, means for producingsawtooth waves occurring at a comparatively low frequency, and means forimpressing said low frequency waves upon a portion of said input circuitwhich is common to both tubes whereby said low frequency waves areimpressed upon said control grids to modulate said high frequency wavesby said low frequency waves and whereby only the modulated waves appearin the secondary of said output transformer.

5. In combination, a pair of electric discharge tubes of the variable mupentode type each having a control grid and a plate, means including aninput transformer and an output transformer for connecting said tubes inpush-pull relation to form a balanced modulator, means for soresistively loading said output transformer that said tubes work into asubstantially purely resistive load having an impedance which is lowcompared with the plate impedance of said tubes whereby said modulatorwill provide linear amplification and linear modulation, means forproducing voltage waves occurring at a comparatively high frequency andfor impressing them upon said input transformer, means for producingsawtooth waves occurring at a comparatively low frequency, and means forvarying the gain of said tubes in phase in accordance with said lowfrequency waves whereby said low frequency waves modulate said highfrequency waves and whereby only the modulated waves appear in thesecondary of said output transformer.

6. In combination, a pair of similar electric discharge tubes eachhaving a control grid and a plate, each of said tubes being of the typewhich has a straight and steep portion on its mutual conductance,control-grid voltage curve, means including an input transformer and anoutput transformer for connecting said tubes in pushpull relation toform a balanced modulator, means for so resistively loading said outputtransformer that said tubes work into a substantially purely resistiveload having an impedance which is low compared with the plate impedanceof said tubes whereby said modulator will provide linear amplificationand linear modulation, means for producing voltage waves occurring at acomparatively high frequency and for impressing them upon said inputtransformer, means for producing saw-tooth waves occurring at acomparatively low frequency, and means for varying the gain of saidtubes in phase in accordance with said low frequency waves whereby saidlow frequency waves modulate said high frequency waves and whereby onlythe modulated waves appear in the secondary of said output transformer.

7. In combination, a pair of electric discharge tubes each having inputelectrodes, a transformer having a primary and a secondary, said tubesbeing connected in balanced relation at said input electrodes by meansof said secondary, an oscillator having a plate circuit, said primarybeing connected in said plate circuit, and a condenser connected inparallel with said primary with respect to said plate circuit wherebysaid condenser and said primary are resonant at a certain frequency, thecapacity and inductance of said condenser and primary, respectively,being such'that said certain frequency is substantially lower than thefrequency at which said oscillator operates. I

8. In combination, a translating device having an input circuit, atransformer having a primary and a secondary, said secondary beingconnected in said input circuit, an oscillator having a plate circuit,said oscillator being of the type which produces electrical impulsesperiodically, said primary being connected in said plate circuit, and acondenser connected in parallel with said primary with respect to saidplate circuit whereby said condenser and said primary form a circuitwhich is resonant at a certain frequency, the capacity and inductance ofsaid condenser and primary, respectively, being such that said cer tainfrequency is substantially lower than the frequency at which saidimpulses occur.

9. In combination, a pair of electric discharge tubes each having inputelectrodes, a transformer having a primary and a secondary; said tubesbeing connected in balanced relation at said input electrodes by meansof said secondary, a blocking oscillator having a plate circuit, saidoscillator being of the type which produces electrical impulsesperiodically, said primary being connected in said plate circuit, and acondenser connected in parallel with said primary with respect to saidplate circuit whereby said condenser and said primary form a circuitwhich is resonant at a certain frequency, the capacity and inductance ofsaid condenser and primary, respectively, being such that said certainfrequency is substantially lower than the frequency at which saidimpulses occur.

10. In combination, a pair of electric discharge tubes connected inbalanced relation to form a balanced translating device having an inputcircuit, an input transformer having a primary and a secondary, saidsecondary being connected in said input circuit, a blocking oscillatorcomprising a tube having inductively coupled plate and grid circuits andcomprising a grid condenser and a grid resistor having such values thatimpulses are produced periodically, said coupling being provided by afeed-back transformer having a primary and a secondary, the primary ofsaid input transformer being connected in said plate circuit in serieswith the primary of said feed-back transformer, and a condenserconnected in parallel with the primary of said input transformer to forma parallel resonant circuit whereby said condenser either charges ordischarges through said oscillator tube upon the occurrence of one ofsaid impulses, the capacity and inductance of said condenser and theprimary of said input transformer, respectively, being such that saidparallel resonant circuit is resonant at a frequency which issubstantially lower than the frequency at which said oscillatoroperates.

11. The invention according to claim 1 characterized in that said firstmeans comprises an oscillator of the type which produces electricalimpulses periodically, said oscillator having a plate circuit, saidinput transformer having a primary connected in said plate circuit, andfurther characterized in that said condenser is connected in parallelwith said primary to form therewith a circuit which is resonant. at afrequency substantially lower than the frequency at which said impulsesoccur.

KARL R. WENDT.

